Sensors that use fiber optics to provide sensor power and/or transmit sensed information are known. These sensors are useful where conventional electrical sensors that use wires to transmit power and information cannot be used due to limitations such as noise susceptibility or the temperature limits of electronics. Optical sensors show promise, as well. Unfortunately, effective use of optical sensors in applications requiring high accuracy and high resolution has been limited to expensive laboratory type equipment.
For example, sensor designers have been unable to create optical sensors that accurately measure small scale physical displacements, particularly micron and sub-micron displacements. Micron displacement measurement, however, is important in applications like flow systems where commonplace silicon micro electromechanical manufacturing system (MEMS) based sensors measure micron displacements in diaphragms. These sensors use various sensing techniques, such as semiconductor strain gage or variable capacitance. In such sensors, the ability to detect micron-level displacements makes it possible to measure flow, pressure, and other physical and material properties with accuracies exceeding 0.01%.
In contrast to silicon sensors, optical sensors using optical interferometry measure micron displacements to no more than 0.1%. Noise in the optical sensor light source, whether laser light or white-light, is a limiting factor since the intensity variations in a single interference band must be accurately measured to provide a high accuracy signal. Imperfections in the optical interferometer mechanisms in these optical sensors also limits sensor accuracy. Larger displacements may be measured with some accuracy, for example by using fringe counting, but these displacements are still larger than those currently sensed with solid-state sensors. Furthermore, optical sensors fail to measure even these larger displacements if the fringe count memory is lost.
Many optical sensors have a Fabry-Perot configuration, using closely-spaced mirrors that define a free-space resonator region. The movable and highly reflective, but partially transmissive mirrors are used to derive a sensed signal. Other laser sensors with a frequency modulated (FM) output have been proposed as a possible solution to the shortcomings of optical sensors. In general, all these devices fall short of addressing the accuracy problems described above. The combination of frequency noise in the laser mechanism and low gage factor (GF) prevent substantially accurate measurements of small scale displacements. Frequency noise, i.e., random drift in operating frequency, limits the resolution of these lasers. Gage factor is a sensitivity measurement and is defined as (fmax−fmin)/fr, where fmax is the output frequency at an upper limit of sensed input, fmin is the output frequency at a minimum level of sensed input signal and fr is the resonant frequency of the system. Low gage factor results in low resolution and undesirable temperature dependence.
A device for modulating laser frequency has been shown having a portion of the laser emission reflected back into the laser from a moving target to effect frequency modulation. The semiconductor diode lasers used exhibit very large frequency noise components, i.e., the base laser frequency varies randomly over a large bandwidth. Further, the external cavity used has a low Q due to limited reflectance from the target. These devices, therefore, are not suitable for measuring small scale displacements in flow systems and other applications.
Another type of laser-based application involves a strain sensing device that utilizes a fiber laser with a cavity defined by Bragg grating reflectors. When strain is imposed on the length of fiber, the lasing frequency of the system changes due to changes in the length of the lasing section. The frequency change that can be measured is limited to the strain that the fiber will withstand, which is typically much less than 0.1%. Furthermore, laser signal drift due to temperature variation and losses in the Bragg reflectors result in loss of accuracy in the measurement of strain.
It is possible to stabilize the frequency of a laser by raising the Q of the mechanism that determines the lasing frequency. In effect, a highly tuned filtering action is achieved which allows only a single frequency to be amplified. This can be achieved by either raising the Q of the lasing cavity itself or by coupling a laser with a low Q cavity to an external cavity with a high Q. A few low noise lasers have been shown in which a high Q micro-cavity, such as a quartz microsphere, emits a stabilized laser signal. These devices, however, have no mechanism for measuring displacement or sensing a physical or material parameter.
As the foregoing indicate, the performance of prior art optical sensors falls below that of the conventional electronic devices that are used in applications like flow systems to measure small scale micron and sub-micron displacements. Thus, while a sensing system based on an optical resonator with a high Q and high gage factor theoretically may provide performance exceeding that of conventional electronic based sensors, none have been shown.